A cyanide-free leaching and reduction precipitation process of gold in waste printed circuit boards based on iodine-iodide system

Abstract

The application discloses a cyanide-free leaching and reduction precipitation process of gold in waste circuit board based on an iodine-iodide system, and the process is characterized in that: first, waste circuit boards are crushed, magnetically separated, eddy current separated and gravity separated to obtain gold-containing materials; then, two-stage cyanide-free leaching is carried out, the pre-leaching solution breaks the organic coating layer by means of a surfactant and N-methyl pyrrolidone, the main leaching solution is based on iodine and iodide as the core, and is combined with an oxidizing agent, a masking agent and a pH buffer agent, and the oxidizing agent concentration is controlled in stages to realize efficient and selective leaching of gold; after the leaching solution is subjected to solid-liquid separation and nanofiltration concentration, a reducing agent is added to realize reduction precipitation to obtain a high-purity gold product; the reduction waste liquid is subjected to electrochemical oxidation regeneration, ion exchange impurity removal and iodide supplement, and then can be recycled as the main leaching solution, so that efficient utilization of reagents and near-zero discharge of waste water are realized. The application has the advantages of fast leaching rate, high selectivity, green and non-toxic, and low reagent consumption, and is suitable for efficient and environmentally-friendly recovery of gold in various waste circuit boards.

Description

Technical Field

[0001] This invention belongs to the field of waste circuit board processing, specifically, it relates to a cyanide-free leaching and reduction precipitation process for gold in waste circuit boards based on an iodine-iodide system. Background Technology

[0002] With the rapid development of the electronics and information industry, the amount of electronic waste generated is increasing year by year, among which waste circuit boards are a significant component. Waste circuit boards contain precious metals such as gold, silver, and palladium, as well as non-ferrous metals such as copper, tin, and aluminum, possessing extremely high resource recycling value. Statistics show that the gold content in each ton of waste circuit boards can reach hundreds to thousands of grams, far exceeding the grade found in primary gold mines. Therefore, efficiently recovering gold from waste circuit boards not only achieves resource recycling but also reduces the environmental pressure caused by primary mineral mining.

[0003] Currently, the mainstream method for extracting gold from waste circuit boards is still the cyanidation method. The cyanidation method utilizes the principle of cyanide ions forming stable complexes with gold, and has advantages such as high leaching rates, mature technology, and low cost. However, the cyanidation method has fatal environmental drawbacks: cyanide is a highly toxic substance, posing significant environmental safety risks during production, storage, transportation, and use; cyanide wastewater is difficult and costly to treat, and leaks can cause serious harm to the ecological environment and human health. With increasingly stringent national environmental regulations, the application of the cyanidation method is facing increasing restrictions.

[0004] To address the environmental concerns associated with cyanide leaching, researchers have developed various cyanide-free gold leaching systems, including the thiosulfate method, the thiourea method, and the halogen method. Among these, the iodine-iodide system is considered one of the most promising cyanide-free gold leaching technologies for industrial application due to its advantages such as fast leaching rate, high selectivity, low toxicity, and minimal equipment corrosion. The basic principle of iodine-iodide gold leaching is as follows: in the presence of an oxidizing agent, iodine is oxidized to elemental iodine or its oxyacid anions, which then oxidize gold and form a stable AuI₂ with iodide ions. - Or AuI4 - The complex enters the solution.

[0005] However, directly applying the iodine-iodide system to the leaching of gold from waste circuit boards still faces the following technical challenges: First, the gold in waste circuit boards is usually tightly encapsulated by materials such as organic resin and epoxy glass cloth. During conventional leaching processes, the leaching agent has difficulty making sufficient contact with the gold, resulting in a slow leaching rate and low leaching yield. Existing technologies mostly use physical methods such as crushing and grinding to expose the gold particles, but physical treatment alone cannot completely remove the organic encapsulation layer, and an organic film remains on the gold surface, hindering the leaching reaction.

[0006] Second, the concentration of oxidant in the iodine-iodide gold leaching system has a significant impact on leaching efficiency. Existing technologies typically employ a constant high concentration of oxidant or add sufficient oxidant at once. However, in actual leaching processes, an excessively high initial oxidant concentration can easily lead to the formation of a dense oxide passivation layer on the gold surface, which in turn inhibits further dissolution of the gold; while an excessively low oxidant concentration makes it difficult to maintain effective complexation and dissolution of the gold.

[0007] Third, the copper content in waste circuit boards is much higher than that of gold. Copper can also form copper-iodine complexes in the iodine-iodide system and enter the solution, resulting in a large amount of copper co-dissolving with gold. On the one hand, the co-dissolving of copper consumes leaching agents, increasing reagent costs; on the other hand, high concentrations of copper impurities can interfere with the subsequent reduction and precipitation of gold, affecting the purity of the gold product. Although existing technologies have attempted to inhibit copper co-dissolving by adding complexing agents or inhibitors, the effect of a single complexing agent in the complex system of waste circuit boards is limited, and the influence of pH changes in the leaching environment on the complexation equilibrium is often ignored.

[0008] Fourth, the leachate suffers from component degradation during recycling. On one hand, the oxidant is reduced during the reaction, and some iodide ions are oxidized to iodate byproducts, leading to an imbalance in the effective iodine-iodide ratio in the leachate. On the other hand, impurity ions such as copper, lead, and tin accumulate in the circulating solution, further reducing leaching selectivity. In addition, pH fluctuations during the leaching reaction affect the stability of the gold-iodine complex and the complexing effect of the complexing agent, resulting in decreased leaching efficiency. Although some existing studies have attempted to improve the process by adding complexing agents, controlling the oxidant concentration, or using recycling, these methods mostly focus on optimizing single aspects and lack a systematic solution.

[0009] To address the aforementioned issues, a highly efficient, stable, and environmentally friendly cyanide-free gold leaching process is urgently needed in this field. This process design aims to simultaneously achieve effective removal of organic coatings from waste circuit boards, passivation inhibition of the gold surface during leaching, effective suppression of impurity metals, and long-term stable circulation of the leaching solution. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention provides a cyanide-free leaching and reduction precipitation process for gold in waste circuit boards based on an iodine-iodide system.

[0011] To achieve the above objectives, the technical solution provided by the present invention is as follows: A cyanide-free leaching and reduction precipitation process for gold in waste circuit boards based on an iodine-iodide system includes the following steps: (1) Pretreatment: The waste circuit boards are crushed and sorted to obtain gold-containing materials enriched with precious metals; (2) Two-stage cyanide-free leaching: First leaching stage: The gold-containing material is placed in a pre-leaching solution for surface activation treatment. The pre-leaching solution contains surfactants and organic solvents, which cause the organic encapsulation layer encapsulating the gold to swell or partially dissolve. Second leaching stage: A main leaching solution is added to the material treated in the first leaching stage to carry out a leaching reaction. The main leaching solution contains iodine, iodide, oxidant, masking agent and pH buffer, so that gold is selectively leached in the form of iodide complex. The oxidant concentration was controlled in stages during the leaching process: the initial oxidant concentration was 2-5 g / L, and when the oxidant concentration was lower than the initial concentration and dropped to 1-2 g / L, oxidant was added to raise the oxidant concentration of the system back to 5-10 g / L. (3) Solid-liquid separation: The leached material is subjected to solid-liquid separation to obtain gold-containing leachate and leaching residue; (4) Reduction precipitation: A reducing agent is added to the gold-containing leaching solution to carry out a reduction reaction, generating elemental gold precipitate, which is then separated into solid and liquid components to obtain the gold product; (5) Recycling: The waste liquid after reduction and precipitation is regenerated by electrochemical oxidation and supplemented with iodide before being returned to step (2) as the main leachate for recycling.

[0012] Furthermore, the pretreatment in step (1) also includes crushing the waste circuit board to a particle size of 0.1~5 mm, and then sequentially subjecting it to magnetic separation, eddy current separation and gravity separation to separate ferromagnetic materials, copper and aluminum metals and non-metallic substrates.

[0013] Furthermore, the pretreatment in step (1) also includes screening and classifying the crushed waste circuit board particles, directly sending the -0.5 mm particle size material into step (2) for leaching, and separating the +0.5 mm particle size material by shaking table gravity separation, and then grinding the gravity separation concentrate to -0.1 mm before entering step (2) for leaching.

[0014] Further, in step (2), the concentration of surfactant in the pre-impregnation solution is 0.5~2 g / L, the concentration of organic solvent is 5~20 mL / L, and the treatment time is 10~30 min.

[0015] Further, in step (2), the iodide is at least one of sodium iodide, potassium iodide, and ammonium iodide; the oxidant is at least one of hydrogen peroxide, sodium persulfate, sodium hypochlorite, sodium chlorate, and manganese dioxide; the masking agent is at least one of ethylenediaminetetraacetic acid, sodium citrate, and sodium tartrate; and the pH buffer is at least one of phosphate, borate, and acetate.

[0016] Further, in step (2), the concentration of iodine in the main leachate is 10~30 g / L, the concentration of iodide is 50~150 g / L, and the concentration of masking agent is 1~5 g / L.

[0017] Further, the reducing agent mentioned in step (4) is at least one of sodium sulfite, sodium bisulfite, sodium metabisulfite, oxalic acid, sodium oxalate, ascorbic acid, iron powder, and zinc powder.

[0018] Furthermore, before reducing the precipitation in step (4), the gold-containing leaching solution is first subjected to nanofiltration concentration treatment to enrich the gold concentration to more than 100 mg / L. At the same time, the enriched iodide solution is separated and directly returned to step (2) as the main leaching solution component.

[0019] Furthermore, during recycling in step (5), ion exchange resin is used periodically to adsorb and remove accumulated copper, lead, and tin impurity ions from the circulating liquid before electrochemical oxidation regeneration.

[0020] Furthermore, the electrochemical oxidation regeneration in step (5) uses a graphite electrode or a titanium-based coated electrode, and the current density is controlled to be 50–200 A / m. 2 .

[0021] Compared with the prior art, the present invention has the following beneficial effects: I. This invention employs a two-stage cyanide-free leaching process. In the first leaching stage, the synergistic effect of surfactants and organic solvents causes the organic encapsulation layer encapsulating gold to swell or partially dissolve, exposing the surface of the gold particles. In the second leaching stage, a strategy of staged control of the oxidant concentration is adopted. Leaching is initiated with a lower concentration to avoid the formation of a dense passivation layer on the gold surface due to excessively high oxidant concentration. Once the oxidant has been consumed to a certain extent, it is replenished to a high concentration to enhance leaching kinetics. This step-by-step design of pre-activation + slow start + enhanced leaching can significantly improve the gold leaching rate and shorten the leaching time.

[0022] II. This invention simultaneously adds a masking agent and a pH buffer to the main leaching solution. The masking agent selectively complexes copper and iron ions in the solution, inhibiting their competition with gold for iodides. Simultaneously, the pH buffer stabilizes the pH of the reaction system, ensuring the complexation efficiency of the masking agent and the stability of the gold-iodine complex. Under these synergistic effects, the co-solubility of copper in the leaching solution is significantly reduced, and the gold product obtained from the reduction precipitation can reach a high purity level, meeting the requirements for high-purity gold materials without further refining.

[0023] Third, this invention constructs a complete recycling system: before reduction precipitation, nanofiltration concentration is used to enrich the gold in the gold-containing leaching solution to a high concentration, while the iodide solution is separated and directly returned to the main leaching solution for use, reducing iodide loss; the waste liquid after reduction precipitation is regenerated through electrochemical oxidation, re-oxidizing iodide ions to elemental iodine, and after adding a small amount of iodide, it is returned to the leaching step; during the recycling process, ion exchange resin is periodically used to adsorb and remove accumulated copper, lead, tin, and other impurity ions, maintaining the selective leaching capacity of the leaching solution. The above closed-loop design enables the leaching solution to be used stably and continuously for a long time, significantly improving the comprehensive utilization rate of iodine and iodides, greatly reducing reagent costs, and achieving near-zero wastewater discharge, which is in line with green and environmentally friendly industrial policies. Detailed Implementation

[0024] The specific embodiments are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Unless otherwise specified, the raw materials and reagents used in the examples are commercially available.

[0025] The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Although the invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the invention without departing from the spirit and scope of the invention, and all such modifications and substitutions should be covered within the scope of the claims of the invention.

[0026] Example 1

[0027] We process 100 kg of waste circuit boards, including computer motherboards.

[0028] (1) Pretreatment: The waste circuit board is crushed to a particle size of 0.5 mm and then subjected to magnetic separation, eddy current separation and gravity separation in sequence to separate ferromagnetic materials, copper and aluminum metals and non-metallic substrates to obtain gold-containing materials enriched with precious metals.

[0029] (2) Two-stage cyanide-free leaching: First leaching stage: The gold-containing material is placed in a pre-leaching solution for surface activation treatment. Pre-leaching solution preparation: Take 0.6 kg of sodium dodecyl sulfate and 6 L of N-methylpyrrolidone, and add water to a total volume of 500 L. Add the gold-containing material to the pre-leaching solution and treat for 20 min to allow the organic encapsulation layer containing the gold to swell and partially dissolve.

[0030] Second leaching stage: The main leachate is added to the material treated in the first leaching stage to initiate the leaching reaction. The main leachate is prepared as follows: 16 kg of iodine, 80 kg of potassium iodide, 9.3 kg of 30% hydrogen peroxide, 2.4 kg of ethylenediaminetetraacetic acid (EDTA), and 4 kg of sodium dihydrogen phosphate are added to a total volume of 800 L. The 9.3 kg of 30% hydrogen peroxide contains 2.8 kg of pure hydrogen peroxide. The main leachate is added to the above material, and the reaction temperature is controlled at 45℃, the stirring speed at 400 rpm, and the pH of the system is stabilized at 7.5 by sodium dihydrogen phosphate. The initial amount of pure hydrogen peroxide added is 2.8 kg. During the reaction, when the residual amount of pure hydrogen peroxide drops to 1.2 kg, 16 kg of 30% hydrogen peroxide (containing 4.8 kg of pure hydrogen peroxide) is added to increase the total amount of pure hydrogen peroxide in the system to approximately 6.0 kg. The reaction continues for a total time of 2.5 h.

[0031] (3) Solid-liquid separation: The leached material is subjected to solid-liquid separation to obtain approximately 780 L of gold-containing leachate and leaching residue. The gold concentration in the gold-containing leachate is approximately 150 mg / L, which is equivalent to approximately 117 g of alloy.

[0032] (4) Reduction and precipitation: The gold-containing leaching solution is first concentrated by nanofiltration to enrich the gold concentration to about 1200 mg / L, yielding about 98 L of concentrated solution. At the same time, about 682 L of permeate, i.e., iodide solution, is separated and directly returned to the preparation of the main leaching solution in step (2). 0.32 kg of sodium sulfite is added to the concentrated solution, which is 1.6 times the theoretical equivalent. The reaction temperature is controlled at 35℃, the stirring speed at 200 rpm, and the pH of the system is adjusted to 5.5. The reaction is carried out for 1.2 h to generate elemental gold precipitate. The gold product is obtained by solid-liquid separation, and the purity of the gold product is not less than 99.9%. At the same time, about 115 L of waste liquid after reduction is obtained.

[0033] (5) Recycling: The waste liquid after reduction is regenerated by electrochemical oxidation: a titanium-coated electrode is used, with a current density of 120 A / m. 2 Electrolysis oxidizes iodide ions to elemental iodine. After electrolysis, 3.2 kg of potassium iodide is added and returned to step (2) as a component of the main leachate for recycling. During the recycling process, the circulating liquid, i.e. the liquid before electrochemical oxidation and regeneration, is periodically taken and passed through an ion exchange resin column. The resin is a strong acidic cation exchange resin with a volume of 50 L to adsorb and remove accumulated copper, lead, and tin impurity ions.

[0034] Example 2

[0035] Processing 100 kg of waste circuit boards, including mobile phone circuit boards.

[0036] (1) Pretreatment: The waste circuit board was crushed to a particle size of 3 mm and then subjected to magnetic separation, eddy current separation and gravity separation to separate ferromagnetic materials, copper and aluminum metals and non-metallic substrates. The crushed particles were screened and classified: about 35 kg of the -0.5 mm particle size was directly fed into the subsequent leaching; about 65 kg of the +0.5 mm particle size was separated by gravity separation on a shaking table to obtain about 18 kg of gravity concentrate, which was then finely ground to -0.1 mm and combined into the leaching.

[0037] (2) Two-stage cyanide-free leaching: First leaching stage: The gold-containing material is placed in a pre-leaching solution for surface activation treatment. Pre-leaching solution preparation: Take 0.25 kg of PEG-400 and 2.5 L of N-methylpyrrolidone, and add water to a total volume of 500 L. Add the gold-containing material to the pre-leaching solution and treat for 10 min to allow the organic encapsulation layer containing the gold to swell and partially dissolve.

[0038] Second leaching stage: The main leachate is added to the material treated in the first leaching stage to initiate the leaching reaction. Preparation of the main leachate: Take 8 kg of iodine, 40 kg of sodium iodide, 1.6 kg of sodium persulfate, 0.8 kg of sodium citrate, and 0.8 kg of sodium borate, and add water to a total volume of 800 L. Add the main leachate to the above material, control the reaction temperature at 30℃, the stirring speed at 200 rpm, and stabilize the pH of the system at 6 using sodium borate. The initial sodium persulfate dosage is 1.6 kg. During the reaction, when the residual sodium persulfate drops to 0.8 kg, add 5.2 kg of sodium persulfate to bring the total sodium persulfate in the system to approximately 6.0 kg. Continue the reaction for a total time of 4 hours.

[0039] (3) Solid-liquid separation: The leached material is subjected to solid-liquid separation to obtain approximately 770 L of gold-containing leachate and leaching residue. The gold concentration in the gold-containing leachate is approximately 120 mg / L, which is equivalent to approximately 92 g of alloy.

[0040] (4) Reduction and precipitation: The gold-containing leaching solution is first concentrated by nanofiltration to enrich the gold concentration to about 1000 mg / L, yielding about 92 L of concentrated solution. At the same time, about 678 L of permeate, i.e., iodide solution, is separated and directly returned to the preparation of the main leaching solution in step (2). 0.11 kg of ascorbic acid is added to the concentrated solution, which is 1.2 times the theoretical equivalent. The reaction temperature is controlled at 20℃, the stirring speed at 150 rpm, and the pH of the system is adjusted to 4. The reaction is carried out for 2 h to generate elemental gold precipitate. The gold product is obtained by solid-liquid separation, and the purity of the gold product is not less than 99.8%. At the same time, about 108 L of waste liquid after reduction is obtained.

[0041] (5) Recycling: The reduced waste liquid is regenerated by electrochemical oxidation: a graphite electrode is used, and the current density is 50 A / m. 2After electrolysis, 1.8 kg of sodium iodide is added and returned to step (2) as a component of the main leachate for recycling. During the recycling process, ion exchange resin is used periodically to adsorb and remove accumulated copper, lead, and tin impurity ions from the circulating liquid before electrochemical oxidation regeneration. The resin used is chelating resin D401 with a volume of 40 L.

[0042] Example 3

[0043] Processing 100 kg of waste circuit boards, including circuit boards for communication equipment.

[0044] (1) Pretreatment: The waste circuit board was crushed to a particle size of 5 mm and then subjected to magnetic separation, eddy current separation and gravity separation to separate ferromagnetic materials, copper and aluminum metals and non-metallic substrates. The crushed particles were screened and classified: about 28 kg of the -0.5 mm particle size was directly fed into the subsequent leaching; about 72 kg of the +0.5 mm particle size was separated by gravity separation on a shaking table to obtain about 22 kg of gravity concentrate, which was then finely ground to -0.1 mm and combined into the leaching.

[0045] (2) Two-stage cyanide-free leaching: First leaching stage: The gold-containing material is placed in a pre-leaching solution for surface activation treatment. Pre-leaching solution preparation: Take 1.0 kg of sodium dodecyl sulfate and 10 L of N-methylpyrrolidone, and add water to a total volume of 500 L. Add the gold-containing material to the pre-leaching solution and treat for 30 min to allow the organic encapsulation layer containing the gold to swell and partially dissolve.

[0046] Second leaching stage: The main leachate is added to the material treated in the first leaching stage to initiate the leaching reaction. Preparation of the main leachate: Take 24 kg of iodine, 120 kg of ammonium iodide, 40 kg of 10% sodium hypochlorite solution, 4 kg of sodium tartrate, and 8 kg of sodium acetate, and add water to a total volume of 800 L. The 40 kg of 10% sodium hypochlorite solution contains 4 kg of available chlorine. Add the main leachate to the above material, control the reaction temperature at 60℃, the stirring speed at 600 rpm, and stabilize the pH of the system at 9 using sodium acetate. The initial amount of available chlorine added is 4 kg. During the reaction, when the residual amount of available chlorine drops to 0.8 kg, add 72 kg of 10% sodium hypochlorite solution, which contains 7.2 kg of available chlorine, to raise the total amount of available chlorine in the system to approximately 8.0 kg. Continue the reaction for a total time of 1 hour.

[0047] (3) Solid-liquid separation: The leached material is subjected to solid-liquid separation to obtain approximately 780 L of gold-containing leachate and leaching residue. The gold concentration in the gold-containing leachate is approximately 180 mg / L, which is equivalent to approximately 140 g of alloy.

[0048] (4) Reduction and precipitation: The gold-containing leaching solution is first concentrated by nanofiltration to enrich the gold concentration to about 1500 mg / L, yielding about 93 L of concentrated solution. At the same time, about 687 L of permeate, i.e., iodide solution, is separated and directly returned to the preparation of the main leaching solution in step (2). 0.28 kg of iron powder is added to the concentrated solution, which is 2.0 times the theoretical equivalent. The reaction temperature is controlled at 50℃, the stirring speed at 300 rpm, the pH of the system is 7, and the reaction is carried out for 0.5 h to generate elemental gold precipitate. The gold product is obtained by solid-liquid separation, and the purity of the gold product is not less than 99.7%. At the same time, about 110 L of waste liquid after reduction is obtained.

[0049] (5) Recycling: The waste liquid after reduction is regenerated by electrochemical oxidation: titanium-coated electrodes are used, with a current density of 200 A / m 2 After electrolysis, 5.0 kg of ammonium iodide is added and returned to step (2) as a component of the main leachate for recycling. During the recycling process, ion exchange resin is used periodically to adsorb and remove accumulated copper, lead, and tin impurity ions from the circulating liquid before electrochemical oxidation regeneration. The resin used is a strong acidic cation exchange resin with a volume of 60 L.

[0050] Example 4

[0051] Processing 100 kg of waste circuit boards, including mixed types of circuit boards.

[0052] (1) Pretreatment: The waste circuit board is crushed to a particle size of 1 mm and then subjected to magnetic separation, eddy current separation and gravity separation in sequence to separate ferromagnetic materials, copper and aluminum metals and non-metallic substrates to obtain gold-containing materials enriched with precious metals.

[0053] (2) Two-stage cyanide-free leaching: First leaching stage: The gold-containing material is placed in a pre-leaching solution for surface activation treatment. Pre-leaching solution preparation: Take 0.75 kg of PEG-600 and 7.5 L of N-methylpyrrolidone, and add water to a total volume of 500 L. Add the gold-containing material to the pre-leaching solution and treat for 25 min to allow the organic encapsulation layer containing the gold to swell and partially dissolve.

[0054] Second leaching stage: The main leachate is added to the material treated in the first leaching stage to initiate the leaching reaction. Preparation of the main leachate: Take 12 kg of iodine, 64 kg of potassium iodide, 3.2 kg of manganese dioxide, 1.6 kg of sodium citrate, and 6.4 kg of disodium hydrogen phosphate, and add water to a total volume of 800 L. Add the main leachate to the above material, control the reaction temperature at 50℃, the stirring speed at 300 rpm, and stabilize the pH of the system at 8 using disodium hydrogen phosphate. The initial manganese dioxide dosage is 3.2 kg. During the reaction, when the residual manganese dioxide drops to 0.9 kg, add 3.6 kg of manganese dioxide to increase the total manganese dioxide content in the system to approximately 4.5 kg. Continue the reaction for a total time of 3 hours.

[0055] (3) Solid-liquid separation: The leached material was subjected to solid-liquid separation to obtain approximately 775 L of gold-containing leachate and leaching residue. The gold concentration in the gold-containing leachate was approximately 140 mg / L, which is equivalent to approximately 108 g of alloy.

[0056] (4) Reduction and precipitation: The gold-containing leaching solution is first concentrated by nanofiltration to enrich the gold concentration to about 1100 mg / L, yielding about 98 L of concentrated solution. At the same time, about 677 L of permeate, i.e., iodide solution, is separated and directly returned to the preparation of the main leaching solution in step (2). 0.22 kg of oxalic acid is added to the concentrated solution, the reaction temperature is controlled at 40℃, the stirring speed is 250 rpm, the pH of the system is adjusted to 6, and the reaction is carried out for 1.5 h to generate elemental gold precipitate. The gold product is obtained by solid-liquid separation, and the purity of the gold product is not less than 99.8%. At the same time, about 115 L of waste liquid after reduction is obtained.

[0057] (5) Recycling: The reduced waste liquid is regenerated by electrochemical oxidation: a graphite electrode is used, and the current density is 100A / m. 2 After electrolysis, 2.5 kg of potassium iodide is added and returned to step (2) as a component of the main leachate for recycling. During the recycling process, ion exchange resin is used periodically to adsorb and remove accumulated copper, lead, and tin impurity ions from the circulating liquid before electrochemical oxidation regeneration. The resin used is chelating resin D401 with a volume of 50 L.

[0058] Comparative Example 1: No pre-impregnation treatment The same waste circuit board and main leaching solution as in Example 1 were used, but the first leaching stage in step (2) was omitted, that is, the gold-containing material was not surface activated and was directly added to the main leaching solution for leaching. The remaining operating conditions were the same as in Example 1.

[0059] Comparative Example 2: Oxidant concentration is constant The same waste circuit board and leaching solution as in Example 1 were used, but in the second leaching stage, the initial oxidant concentration was directly set to 8 g / L without phased replenishment, meaning a constant high concentration of oxidant was maintained throughout the process. All other operating conditions were the same as in Example 1.

[0060] Comparative Example 3: No masking agent and pH buffer The same waste circuit board and leachate composition as in Example 1 were used, but no masking agent or pH buffer was added to the main leachate, and the other operating conditions were the same as in Example 1.

[0061] Comparative Example 4: No recycling The same waste circuit board and leachate were used as in Example 1, but in step (5), the waste liquid after reduction was not electrochemically oxidized and regenerated, nor was ion exchange resin used to remove impurities. That is, freshly prepared leachate was used for each leaching, and the waste liquid was directly discharged or treated separately. The remaining operating conditions were the same as in Example 1.

[0062] The data from Examples 1-4 and Comparative Examples 1-4 were compared, and the specific data are shown in the table below: Note: Comparative Examples 1-4 are all based on the process conditions of Example 1, lacking only a single feature, and are used to qualitatively illustrate the necessity of each technical feature. Data from Examples 2-4 are also listed to demonstrate the feasibility of the invention within different parameter ranges. Analysis of the table data shows: 1. Gold leaching rate Examples 1-4 all achieved a leaching rate of over 97%, with Example 3 having the highest and Example 2 the lowest. Both were significantly higher than Comparative Examples 1 and 2, slightly higher than Comparative Example 3, and on par with Comparative Example 4, indicating that pre-immersion activation and staged control of the oxidant are crucial for improving the leaching rate.

[0063] 2. Immersion time Example 3 requires only 1 hour, and Example 2 requires 4 hours, both being optimal under their respective conditions. Comparative Example 1, due to the lack of pre-soaking, requires 4.5 hours, and Comparative Example 2, due to passivation, requires 3.2 hours, indicating that the leaching rate of the present invention can be adjusted as needed, and both are superior to schemes lacking key features.

[0064] 3. Purity of gold products Examples 1-4 all achieved a purity of over 99.7%, with Example 1 showing the highest purity. Comparative Example 3, lacking both masking agent and pH buffer, had a purity of only 92.5%. The other comparative examples had similar purities, indicating that the synergistic effect of the masking agent and pH buffer significantly improved product purity.

[0065] 4. Copper co-solubility In Examples 1-4, the copper co-solubility was less than 10%, with Example 2 showing the lowest. Comparative Example 3, however, had a high rate of 45.6%, demonstrating that the masking agent and pH buffer effectively inhibited copper leaching. This indicates that the present invention significantly reduces impurity co-solubility and alleviates the burden of subsequent purification.

[0066] 5. Iodide consumption per unit The unit consumption of Examples 1-4 was between 11.8 and 13.2 kg / t. Comparative Example 2 increased to 15.6 kg / t due to oxidant waste, Comparative Example 3 increased to 18.3 kg / t due to copper consumption, and Comparative Example 4 had a unit consumption as high as 86.4 kg / t without recycling, indicating that recycling significantly reduced reagent consumption, and that staged regulation and masking agents also contributed to the reduction.

[0067] 6. Wastewater discharge volume Examples 1-4 and Comparative Examples 1-3 all achieved recycling, with wastewater discharge of only 45-55 L / t. Comparative Example 4 had no recycling, with a discharge of 850 L / t, indicating that closed-loop recycling achieved near-zero emissions.

[0068] 7. Recycled batches Examples 1-4 and Comparative Examples 1-3 can be recycled more than 50 times, while Comparative Example 4 can only be recycled once, indicating that electrochemical regeneration and ion exchange maintenance ensure the long-term stability of the leachate.

[0069] In summary, Examples 1-4 all achieve excellent technical results within their respective selected parameter ranges, with leaching rates ≥97.2%, product purity ≥99.7%, copper co-solubility ≤9.1%, iodide consumption ≤13.2 kg / t, wastewater discharge ≤55 L / t, and recycling batches >50. Compared with Comparative Examples 1-4, which lack any key feature, this invention has significant advantages in leaching rate, leaching time, product purity, reagent consumption, and environmental friendliness. This demonstrates that the systematic design of pre-leaching activation, staged regulation of oxidant, synergistic use of masking agent and pH buffer, and closed-loop recycling is indispensable and works synergistically to produce unexpected technical effects.

[0070] It should be understood that the above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. It should not be considered that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A cyanide-free leaching and reduction precipitation process for gold in waste circuit boards based on an iodine-iodide system, characterized in that, Includes the following steps: (1) Pretreatment: The waste circuit boards are crushed and sorted to obtain gold-containing materials enriched with precious metals; (2) Two-stage cyanide-free leaching: First leaching stage: The gold-containing material is placed in a pre-leaching solution for surface activation treatment. The pre-leaching solution contains surfactants and organic solvents, which cause the organic encapsulation layer encapsulating the gold to swell or partially dissolve. Second leaching stage: A main leaching solution is added to the material treated in the first leaching stage to carry out a leaching reaction. The main leaching solution contains iodine, iodide, oxidant, masking agent and pH buffer, so that gold is selectively leached in the form of iodide complex. The oxidant concentration was controlled in stages during the leaching process: the initial oxidant concentration was 2-5 g / L, and when the oxidant concentration was lower than the initial concentration and dropped to 1-2 g / L, oxidant was added to raise the oxidant concentration of the system back to 5-10 g / L. (3) Solid-liquid separation: The leached material is subjected to solid-liquid separation to obtain gold-containing leachate and leaching residue; (4) Reduction precipitation: A reducing agent is added to the gold-containing leaching solution to carry out a reduction reaction, generating elemental gold precipitate, which is then separated into solid and liquid components to obtain the gold product; (5) Recycling: The waste liquid after reduction and precipitation is regenerated by electrochemical oxidation and supplemented with iodide before being returned to step (2) as the main leachate for recycling.

2. The process according to claim 1, characterized in that: The pretreatment described in step (1) also includes crushing the waste circuit board to a particle size of 0.1~5 mm, and then sequentially subjecting it to magnetic separation, eddy current separation and gravity separation to separate ferromagnetic materials, copper and aluminum metals and non-metallic substrates.

3. The process according to claim 1, characterized in that: The pretreatment described in step (1) also includes screening and classifying the crushed waste circuit board particles, directly sending the -0.5 mm particle size material into step (2) for leaching, and separating the +0.5 mm particle size material by shaking table gravity separation, and then grinding the gravity separation concentrate to -0.1 mm before entering step (2) for leaching.

4. The process according to claim 1, characterized in that: In step (2), the concentration of surfactant in the pre-impregnation solution is 0.5~2 g / L, the concentration of organic solvent is 5~20 mL / L, and the treatment time is 10~30 min.

5. The process according to claim 1, characterized in that: The iodide mentioned in step (2) is at least one of sodium iodide, potassium iodide, and ammonium iodide; the oxidant is at least one of hydrogen peroxide, sodium persulfate, sodium hypochlorite, sodium chlorate, and manganese dioxide; the masking agent is at least one of ethylenediaminetetraacetic acid, sodium citrate, and sodium tartrate; and the pH buffer is at least one of phosphate, borate, and acetate.

6. The process according to claim 1, characterized in that: In step (2), the concentration of iodine in the main leachate is 10-30 g / L, the concentration of iodide is 50-150 g / L, the concentration of masking agent is 1-5 g / L, and the concentration of pH buffer is 1-10 g / L.

7. The process according to claim 1, characterized in that: The reducing agent mentioned in step (4) is at least one of sodium sulfite, sodium bisulfite, sodium metabisulfite, oxalic acid, sodium oxalate, ascorbic acid, iron powder, and zinc powder.

8. The process according to claim 1, characterized in that: Before reducing the precipitation in step (4), the gold-containing leaching solution is first concentrated by nanofiltration to enrich the gold concentration to more than 100 mg / L. At the same time, the enriched iodide solution is separated and directly returned to step (2) as the main leaching solution component.

9. The process according to claim 1, characterized in that: In step (5), during recycling, ion exchange resin is used to periodically adsorb and remove accumulated copper, lead, and tin impurity ions from the circulating liquid before electrochemical oxidation regeneration.

10. The process according to claim 1, characterized in that: The electrochemical oxidation regeneration described in step (5) uses a graphite electrode or a titanium-coated electrode, and the current density is controlled to be 50–200 A / m. 2 .

Images